EP-A 587 238 has disclosed a metal halide lamp with a ceramic discharge vessel, in which a two-part leadthrough is sealed in an elongate stopper capillary by means of soldering glass at the end of the stopper which is remote from the discharge. The outer part of the leadthrough consists of permeable material (niobium pin), while the inner part consists of halide-resistant material (for example a pin made from tungsten or molybdenum). For relatively high lamp outputs (up to approximately 400 W), a different solution is used, namely that of replacing the inner Mo pin part by a cermet part. The coefficient of thermal expansion of this part can be adjusted as desired between that of other metal parts and that of the ceramic.
A drawback of solutions of this type is that the connection between the inner part of the leadthrough and the electrode is very prone to break. This is true both during further processing of the electrode system and during the service life of the system while the lamp is operating. Electrodes which bend can ultimately lead to the discharge vessel exploding during operation.
WO 01/82331 attempts to avoid this by using a multipart arrangement for the leadthrough. However, this represents only an inadequate solution to the basic problem. The diameter of the electrode is generally less than that of the inner part, the two components being connected by fusing the end of the inner part and embedding the end of the electrode in it. The fusing operation is often effected by brazing or laser soldering. The inner part usually consists of molybdenum or Mo-containing cermet. In this case, however, the amount of fusion at the inner part cannot be ensured reproducibly within the required level of accuracy. A remedy to this would be to increase the fusible length. However, this encounters problems with the limited maximum permissible “welding production height”. What this means is an increase in height which results from a local accumulation of weld metal or solder in the region of the welding or soldering zone. It may also be slag (in particular in the case of a cermet connection). The maximum permissible degree of the increase in height is in this case determined by the minimum permissible capillary internal diameter of the discharge vessel.